This project concerns two aspects of mouse specific thalamic relay cells (TCRs) from the whisker "barreloids" in the ventroposterior medial nucleus (VPM) to their targets the whisker "barrels" in the somatosensory cortex; specifically the denfrites and axon terminasl (thalamocortical afferents+TCAs) of these neurons. General Hypothesis I. TCR dendrites define different roles in development and maturity: Studies of TCR dendrite organization in VPM. We suggest that an early TCR role is to transmit pattern information from the brainstem to the cortex; a later rrole is to incorporate information from different whiskers. TCRs in fixed slices through VPM will be stained by intracellular injections of dye and reconstructed and measured with a computer microscope to: 1) determine directly the relationship of TCR dendrites to segregated afferents from brainstem trigeminal nuclei; 2) follow the postnatal development of TCR dendrites including documentating functional correlates of anatomical changes; and, 3) demonstrate the TCR dendrites are plastic and can be changed by altering the class or pattern of inputs to VPM from the brainstem. General hypothesis II. TCA grouping in development is a population phenomenon; Studies of TCA organization in barrel cortex. We suggest that the rich and dense afferent patterns from the barreloids to the barrels arises in part from interactions with each other. TCAs will be stained in bulk and signly by dyes in living and fixed materials and analyzed with quantitative confocal and ocmputer microscopy to; 1) determine the extent to which single fibers and groups of fibers to particular barrels share the same characteristics; 2) follow the development of the clustering of these fibers in early postnatal life; and, 30 demonstrate differences and similarities between fibers supplying the cortex of animals deafferented since bith and those where the pattern of TCA inputs is perturbed by selective whisker removal. The project will use the St. Louiis CORE for intracellular staining, three dimensional measurements of processes and for quantitation of TCAs. We will rely on the expertise of many of the other investigators in this project for advice, guidance and assistance with targeting markers, recording and collecting and analyzing the data. This project is related to higher-order processing of trigeminal information and its disturbances with environmental modifications to the nervous system.